The art of resistance welding has advanced to the point where someone skilled in the art can set a limited number of welding parameters to produce high quality welds for a given weld condition. The American Welding Society and the Resistance Welding Manufacturer's Association have both established recommended practices which are guidelines for the production of high quality welds. Thus, for a given weld condition specifying sheet thickness, material, coating, electrode geometry and type, the guideline will provide a range of electrode force, current level, weld time, and weld spacing and will give the expected weld nugget size and weld shear strength. In practice, however, this process frequently fails to produce high quality welds consistently because of the presence of a large number of often difficult to control variables. Some of these variables are:
1. Coating material
the type of coating as well as surface contaminants such as paint or grease can cause large changes in the nugget formation time given the same control timing and current PA1 this can cause very rapid cap wear due to arcing in the case of pressure drop PA1 this can also change the time to form a good nugget by a factor of two PA1 this can cause very rapid cap wear due to arcing PA1 this can cause uncontrolled nugget formation PA1 this can increase the time to produce a good weld by a factor of two or more given the same current
2. Galvanize coating thickness variations
3. Air pressure variations
4. Air cylinder wear
5. Rising tip force during a weld
6. Operator changes in programmed squeeze time
Thus, the recommended practices do not insure good welds under factory conditions where variables can occur. It is desirable, of course, to be able to produce welds in a factory environment with a high degree of confidence in the weld quality. In the automotive industry it is desirable to produce good welds using minimum weld time.
It is a known practice, as disclosed in the Cohen U.S. Pat. No. 4,447,700 to attempt to render the weld process more uniform by applying an initial preheat current pulse for conditioning the workpiece to remedy fit-up and surface problems. It is also known, as disclosed in Schumacher et al. U.S. Pat. No. 4,456,810 to precondition the workpiece by applying low magnitude current pulses after full electrode force is applied, followed by a cooling period, which results in a longer process cycle. The success of these practices is limited, at best, although welds may have been improved in some cases. Consistent improvement has not been realized because heretofore the precise requirements for a preheat pulse have not been known. To be successful it is required that the preheat pulse stabilize the weld process with no sacrifice in weld time and preferably effect a time savings. In the interest of time economy there have been attempts to start current just after the electrodes clamp the workpiece. To accomplish this the welder is set up to apply current at a set time, but the process variations can cause undesired events such as arcing and slow weld nugget development.